1. Field of the Invention
The present invention relates to the design of passive optical networks. More specifically, the present invention relates to a method and apparatus for discovering multiple remote nodes in an Ethernet passive optical network.
2. Related Art
In order to keep pace with increasing Internet traffic, optical fibers and associated optical transmission equipment have been widely deployed to substantially increase the capacity of backbone networks. However, this increase in the capacity of backbone networks has not been matched by a corresponding increase in the capacity of access networks. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks creates a severe bottleneck in delivering high bandwidth to end users.
Among the different technologies presently under development, the Ethernet passive optical network (EPON) is one of the best candidates for next-generation access networks. EPONs combine ubiquitous Ethernet technology with inexpensive passive optics. Hence, they offer the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. In particular, due to the high bandwidth of optical fibers, EPONs are capable of accommodating broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, since Ethernet frames can directly encapsulate native IP packets with different sizes, whereas ATM passive optical networks (APONs) use fixed-size ATM cells and consequently require packet fragmentation and reassembly.
Typically, EPONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and business or residential subscribers. Logically, the first mile is a point-to-multipoint network, with a central office servicing a number of subscribers. A tree topology can be used in an EPON, wherein one fiber couples the central office to a passive optical splitter, which divides and distributes downstream optical signals to subscribers and combines upstream optical signals from subscribers (see FIG. 1).
Transmissions within an EPON are typically between an optical line terminal (OLT) and optical networks units (ONUs) (see FIG. 2). Note that ONUs are sometimes also called optical network terminals (ONTs). The OLT generally resides in the central office and couples the optical access network to a metro backbone, which is typically an external network belonging to an Internet Service Provider (ISP) or a local exchange carrier. An ONU can be located either at the curb or at an end-user location, and can provide broadband voice, data, and video services. ONUs are typically coupled to a one-by-N (1×N) passive optical coupler, where N is the number of ONUs, and the passive optical coupler is typically coupled to the OLT through a single optical link. This configuration can achieve significant savings in the number of fibers and amount of hardware required by EPONs.
Communications within an EPON include downstream traffic (from OLT to ONUs) and upstream traffic (from ONUs to OLT). In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, downstream data frames broadcast by the OLT can reach all ONUs and are subsequently extracted by their destination ONUs. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler with the OLT.
Correspondingly, an EPON typically employs some arbitration mechanism to avoid data collision and to provide fair sharing of the upstream fiber-channel capacity. This is achieved by allocating a transmission window (timeslot) to each ONU. Each timeslot is capable of carrying several Ethernet packets. An ONU typically buffers packets it receives from a subscriber until its timeslot arrives. When its timeslot arrives, the ONU “bursts” all stored frames to the OLT at full channel speed.
To allow ONUs to join an EPON at arbitrary times, an EPON generally has two modes of operation: a discovery (initialization) mode and a normal operation mode. The normal operation mode accommodates regular upstream data transmissions, in which transmission opportunities are assigned in turn to all initialized ONUs. Note that an OLT regularly enters discovery mode to allow new ONUs to join the EPON. The discovery mode provides a time window used to detect newly joined ONUs while regular upstream data transmission is suspended. When the OLT successfully registers a newly joint ONU, it assigns the ONU a logical link ID (LLID), which corresponds to the ONUs medium access control (MAC) address. This LLID identifies the particular ONU during future communications with the OLT
The current EPON architecture allows an ONU to have multiple LLIDs. When an OLT assigns multiple LLIDs to an ONU, each LLID represents an equivalent of a logical ONU to the OLT, although all the LLIDs are associated to a common physical ONU. This feature allows a number of user devices to couple with the common physical ONU and operate as virtual ONUs (VONUs). During discovery, these VONUs may behave just like a regular ONU, although they transmit all their data through the common physical ONU.
Because more than one unregistered VONU can request registration with the OLT and the upstream response messages are not scheduled (because the newly joined VONUs are not initialized yet), the discovery process is subject to collision between response messages. If the collision probability is high, an EPON will need to stay in discovery mode for a longer time and may need to enter discovery mode more frequently, resulting in reduced usable bandwidth for regular data transmission.
Hence, what is needed is a method and apparatus for discovering remote nodes in an EPON, which reduces collision during the discovery process and provides more efficient upstream bandwidth utilization.